Nanostructures are structures that have sizes ranging from about 0.5 nanometers (nm) to about 1 micrometer (μm). Nanostructures exist in a wide variety of different forms, including nanoparticles and nanotubes. As used herein, the term “nanoparticle” refers to a physical composition of matter characterized by a size (e.g., diameter) ranging from about 0.5 nm to about 100 nm. The term “nanotube” refers to an elongated hollow or solid structure having a cross section or diameter less than 1 μm. Carbon nanotubes, for example, typically are hollow graphite tubules that typically have diameters ranging on the order of about 1–50 nm. Carbon nanotubes typically have rigid three-dimensional carbon structures that have high surface areas, low bulk density, and high crush strength.
Nanoparticles in the size range of 1–50 nm have been attached to substrates for a variety of purposes, including many applications that leverage the catalytic properties of certain nanoparticles. Many techniques generate surfaces that are coated with a random distribution of nanoparticles. For example, Klinke et al. (“Thermodynamic calculations on the catalytic growth of carbon nanotubes,” AIP Conf. Proc. 685(1) 447 (20 Oct. 2003)), describe a thin film formed by dipping or spin-casting of Fe(NO3)3 dissolved in propanol can then be thermally treated so that it forms small Fe2O3 nanoparticles. Zhang et al. (“Imaging as-grown single-walled carbon nanotubes originated from isolated catalytic nanoparticles,” Appl. Phys. A, Vol. 74, 325–328 (2002)) describe the use of hollow proteins such as ferritin to capture and store Fe species and subsequently form ferric oxide nanoparticles by removing the host protein. Kong et al. (“Synthesis of individual single-walled carbon nanotubes on patterned silicon wafers,” NATURE, Vol. 395, 29 Oct. 1998, pp 878 ff.) describe mixing Fe(NO3)3 solution with Al2O3 nanoparticles and forming iron oxide nanoparticles onto the Al2O3 matrix. In all of these methods the placement of the iron-containing nanoparticles is random.
Other nanoparticle generation techniques have been proposed in which the locations of the nanoparticles on a surface are controlled with lithographic precision. For example, U.S. Pat. No. 6,346,189 describes a method of forming carbon nanotubes on catalyst islands. The catalyst islands are formed by exposing an underlying substrate through holes etched in a photoresist layer. The holes are about 3–5 μm in size and are spaced apart by a distance of about 10 μm. A solution of Fe(NO3)3 in methanol mixed with alumina nanoparticles about 15–30 nm in size is deposited on the photoresist and the surface areas of the substrate exposed by the holes. A lift-off process is performed to leave isolated islands of Fe(NO3)3-coated alumina nanoparticles adhering to regions of the substrate that were exposed by the holes in the photoresist. The substrate is heated to decompose the Fe(NO3)3 into Fe2O3. Single-walled nanotubes are formed by heating the substrate and exposing the catalyst islands to pure methane at a temperature of about 850–1000° C.